Tropical Storm Don unimpressive so far

Tropical Storm Don formed yesterday from an African tropical wave that crossed into the Gulf of Mexico, and the thus-far unimpressive storm appears poised to bring tropical storm conditions to the lower Texas coast by Friday night. An Air Force Hurricane Hunter plane arrived in the center of Don around 8am EDT this morning, and has found Don to be a small tropical storm with top winds near 40 mph. The 7:57am EDT center fix found a central pressure of 1002 mb, which is 2 mb higher than NHC was estimating in its 8am EDT advisory. However, a pass through the center at 9:49am EDT found the pressure had dropped 2mb, to 1000 mb. Top reliable surface winds seen by the Air Force plane with its SFMR instrument as of 9:45am EDT were 41 mph, at 8:10 am EDT. Water vapor satellite images show a region of dry air to the northwest of Don, over the western Gulf of Mexico. Wind shear as diagnosed by the University of Wisconsin CIMSS group and the SHIPS model show a moderate 10 - 15 knots of wind shear. The shear is from strong upper level winds out of the north. Since the atmosphere to the north of Don is relatively moist, the moderate shear will not be as damaging to the storm as if these winds had been blowing from the northwest, where the driest air lies. Thus the shear direction is often just as important as the strength of the shear, and in Don's case, the shear direction should not force significant amounts of storm-disrupting dry air into the core. Water temperatures in the Gulf of Mexico are near 29°C, which is 2.5°C above the 26.5°C threshold typically needed to maintain a tropical storm.

Figure 1. Morning satellite image of Don.

Satellite imagery shows that Don is a very small storm. Thus, the storm is vulnerable to pockets of dry air and modest-sized jets of wind shear that we can't see from the relatively coarse-resolution data collected by surface stations, hurricane hunter flights, and satellites. The moderate wind shear over Don is keeping Don's circulation tilted so that the surface center is displaced from the center at higher levels. This tilt is keeping the storm from intensifying. Latest visible satellite loops show a modest increase in the intensity of the thunderstorm's near Don's center began at 9am EDT, but this could be a transient burst and not a sign the storm is undergoing intensification.

Figure 2. The latest drought map for Texas shows that over 75% of the state is in exceptional drought--the highest category of drought. Image credit: U.S. Drought Monitor.

Forecast for DonThe big question for Don is, will it be a boon or bane for Texas? The state is currently suffering through its worst drought in recorded history, and Don has the potential to bring some decent drought-busting rains to the state. If Don can expand in size and intensify to a 50 - 55 mph tropical storm, it has the capability to bring hundreds of millions of dollars worth of beneficial rains to the state. However, we have a Goldilocks problem. We can't have Don intensifying into a hurricane, or its winds and flooding might bring hundreds of millions in damage. Neither do we want Don to stay in its current state, which is too small and weak to bring significant rains to Texas. If Don follows the current NHC forecast, which brings the storm up to a moderate-strength tropical storm, that would be just right. This forecast is low-confidence, though, since Don's small size makes it prone to sudden changes in strength, either upward or downward. NHC is giving Don just a 14% chance of intensifying into a hurricane in its 5am advisory, but this could easily change upward if Don manages to overcome its vertical tilt and start consolidating an eyewall. I put the odds of Don reaching hurricane strength at 30%. None of the computer models is predicting Don will become a hurricane.

A small system like Don is relatively difficult to resolve in some of the computer models we use to forecast tropical storm track, and the forecast tracks of Don from these models have a higher spread than usual. For those of you wondering about your odds of experiencing tropical storm force winds, I recommend NHC's wind probability forecast, which is showing that Corpus Christi and Port O'Connor, Texas have the highest chance of 39+ mph winds: 40%.

Quoting NotCircumventing:Will have to look at this same map in a few minutes for the 1500 update ... but I think the ridge is eroding just a hair in the NW gulf, enough to allow Don to steer more on a true NW heading of 315 (or so), with ultimate landfall near Matagorda, TX (or so)

Been looking at that this morning, and I just don't see the westward push. Step backward in time on those charts and you see that the trough over the AZ area was apparently weaker. Now it looks a little stronger, and the western edge of the ridge looks to be eroding slightly.

Had to go do an appraisal on the Bolivar after Ike and it was sad to see all of the destruction. We go to Galveston at least once a year and each time we go back it gets better and better. There are still buildings left to ruin but more and more are being demolished for new homes or commercial sites. I'll have to remember to takes some pictures next time I go. And I'll see if I still have those originals from 2008 too.

Thanks....I was part of the relief effort for that storm based out of Seguin, TX

THE INITIAL MOTION IS NOW 305/12. DON IS TO THE SOUTH AND SOUTHWESTOF A DEEP-LAYER RIDGE OVER THE SOUTHEASTERN UNITED STATES...AND THELARGE-SCALE MODELS FORECAST THIS FEATURE TO PERSIST FOR THE NEXT36-48 HR. THIS SHOULD STEER DON GENERALLY NORTHWESTWARD TO WEST-NORTHWESTWARD UNTIL LANDFALL ON THE TEXAS COAST BETWEEN 36-48 HR. WHILE THE TRACK GUIDANCE AGREES ON THIS SCENARIO...THERE HAS BEEN ASIGNIFICANT NORTHWARD SHIFT IN THE GUIDANCE SINCE THE PREVIOUSADVISORY...WITH THE CENTER OF THE GUIDANCE ENVELOPE NOW NORTH OFCORPUS CHRISTI. THE NEW TRACK FORECAST WILL ALSO SHIFT NORTHWARDBUT STAYS TO THE SOUTH OF THE CENTER OF THE NEW GUIDANCE ENVELOPE. IT SHOULD BE NOTED THAT THE ECMWF AND UKMET FORECAST TRACKS ARE ONTHE SOUTHERN EDGE OF THE GUIDANCE ENVELOPE TO THE SOUTH OF THECURRENT FORECAST TRACK.

THE INITIAL MOTION IS NOW 305/12. DON IS TO THE SOUTH AND SOUTHWESTOF A DEEP-LAYER RIDGE OVER THE SOUTHEASTERN UNITED STATES...AND THELARGE-SCALE MODELS FORECAST THIS FEATURE TO PERSIST FOR THE NEXT36-48 HR. THIS SHOULD STEER DON GENERALLY NORTHWESTWARD TO WEST-NORTHWESTWARD UNTIL LANDFALL ON THE TEXAS COAST BETWEEN 36-48 HR. WHILE THE TRACK GUIDANCE AGREES ON THIS SCENARIO...THERE HAS BEEN ASIGNIFICANT NORTHWARD SHIFT IN THE GUIDANCE SINCE THE PREVIOUSADVISORY...WITH THE CENTER OF THE GUIDANCE ENVELOPE NOW NORTH OFCORPUS CHRISTI. THE NEW TRACK FORECAST WILL ALSO SHIFT NORTHWARDBUT STAYS TO THE SOUTH OF THE CENTER OF THE NEW GUIDANCE ENVELOPE. IT SHOULD BE NOTED THAT THE ECMWF AND UKMET FORECAST TRACKS ARE ONTHE SOUTHERN EDGE OF THE GUIDANCE ENVELOPE TO THE SOUTH OF THECURRENT FORECAST TRACK.

Id love to see some pictures of Galveston since 2008. Let me know if you have any...

Had to go do an appraisal on the Bolivar after Ike and it was sad to see all of the destruction. We go to Galveston at least once a year and each time we go back it gets better and better. There are still buildings left to ruin but more and more are being demolished for new homes or commercial sites. I'll have to remember to takes some pictures next time I go. And I'll see if I still have those originals from 2008 too.

Not a Met and been lurking this last week. Is the high supposed to weaken and move off to let Don go more northward? Been hearing off and on that it would and then again yesterday got the impression it was stronger than thought. Now weaker again. I'm one of those SWLA peeps and would like a little more rain (wishcasting) if it came closer to Houston/Port Arthur area - NE quadrant wisher.

"Did you know that...On this date in 1609, a hurricane tracking near the Bahama Islands crippled The Sea Venture, forcing the people aboard to take shelter on the islands. The captain initially named the islands Somers Islands after himself. Most of the colonists moved to Virgina the next year, but the island still celebrates Somers Day on this date."